The present invention relates to a process for producing gasoline with a low sulphur content, which enables the whole of a sulphur-containing gasoline cut to be upgraded, and to reduce the total sulphur and mercaptan content of said gasoline cut to very low levels with no substantial drop in the gasoline yield, while minimising the octane number reduction.
The production of reformulated gasolines satisfying the latest environmental specifications primarily requires reducing the concentration of olefins and/or aromatics (especially benzene) and sulphur (including mercaptans). Catalytically cracked gasoline has a high olefin content, and the sulphur present in reformulated gasoline in particular is attributable to an extent of almost 90% to catalytic cracking gasoline (FCC, fluid catalytic cracking, or fluidised bed catalytic cracking). Desulphurising (hydrodesulphurising) gasoline and principally FCC gasoline is thus of clear importance.
Hydrotreating (hydrodesulphurising) the feed sent for catalytic cracking results in gasoline which typically contains 100 ppm of sulphur. Units for hydrotreating catalytically cracked feeds operate, however, under severe temperature and pressure conditions, which thus means a substantial investment. Further, the whole of the feed has to be desulphurised, which means that very large volumes of feed have to be treated.
When carried out under conventional conditions which are known to the skilled person, hydrotreating (or hydrodesulphurising) catalytic cracking gasoline, can reduce the sulphur content of the cut. However, that process has the major disadvantage of causing a very substantial drop in the octane number of the cut because of saturation of all of the olefins during the hydrotreatment.
Separating a light and a heavy gasoline before hydrotreating has already been claimed in United States patent US-A-4 397 739. That patent claims a process for hydrodesulphurising gasoline comprising fractionating the gasoline into a light fraction and a heavy fraction and specific hydrodesulphurisation of the heavy fraction.
Further, US-A-4 131 537 teaches that it is important to fractionate the gasoline into several cuts, preferably three, as a function of their boiling point, and desulphurising them under conditions which can be different. That patent indicates that the major benefit is obtained when the gasoline is fractionated into three cuts and when the cut with intermediate boiling points is treated under mild conditions.
European patent application EP-A-0 725 126 describes a process for hydrodesulphurising a cracking gasoline in which the gasoline is separated into a plurality of fractions comprising at least a first fraction which is rich in compounds which are easy to desulphurise and into a second fraction which is rich in compounds which are difficult to desulphurise. Before carrying out that separation, the distribution of the sulphur-containing products must be determined by analysis. Such analyses are necessary to be able to select the apparatus and the separation conditions.
That application also claims that the olefin content and octane number of a light fraction of cracked gasoline fall substantially when it is desulphurised without being fractionated. In contrast, fractionation of that light fraction into 7 to 20 fractions followed by analyses of the sulphur contents and olefin contents of those fractions enables the fraction or fractions which are richest in sulphur- containing compounds to be determined, which fractions are then desulphurised simultaneously or separately and mixed with other desulphurised or non desulphurised fractions. Such a procedure is complex and must be repeated every time the composition of the gasoline to be treated is changed.
It is also important to note that compounds which are termed xe2x80x9ceasyxe2x80x9d to desulphurise are particularly, as indicated in application EP-A-0 725 126, benzothiophene and methylbenzothiophene which have boiling points of 220xc2x0 C. and 244xc2x0 C. respectively. Such compounds are thus in the xe2x80x9chigh boiling pointxe2x80x9d cut of US-A-4 131 537, which according to that patent requires the most severe treatment for desulphurisation.
US-A-5 290 427 has proposed processes for hydrotreating gasoline consisting of fractionating the gasoline then desulphurising the fractions and converting the desulphurised fractions on a ZSM-5 zeolite to compensate for the octane number loss by isomerisation.
US-A-5 318 690 proposes a process with fractionation of the gas and sweetening of the light 25 fraction, while the heavy fraction is desulphurised then converted on ZSM-5 and desulphurised again under mild conditions. That technique is based on separating the unrefined gasoline so as to obtain a light cut which is practically free of sulphur-containing compounds other than mercaptans. That enables the cut to be treated solely with a sweetening method which removes the mercaptans.
For this reason, the heavy cut contains a relatively large quantity of olefins which are partially saturated during hydrotreatment. To compensate for the drop in octane number connected with olefin hydrogenation, the patent recommends cracking on ZSM-5 zeolite which produces olefins, but to the detriment of the yield. Further, such olefins can recombine with the H2S present in the medium to reform the mercaptans. Sweetening or supplemental hydrodesulphurisation must then be carried out.
The present invention relates to a process for producing gasoline with a low sulphur content, which enables the whole of a sulphur-containing gasoline cut to be upgraded, and to reduce the total sulphur and mercaptan content of said gasoline cut to very low levels, with no substantial drop in the gasoline yield, while minimising the octane number reduction.
The process of the invention is a process for producing a gasoline with a low sulphur content from a sulphur-containing gasoline cut. The process of the invention comprises separating said gasoline into a light fraction and a heavy fraction, hydrodesulphurising the light gasoline on a nickel-based catalyst, hydrodesulphurising the heavy fraction on a catalyst comprising at least one group VIII metal and/or at least one group VIB metal, and mixing the desulphurised fractions.
The feed for the process of the invention is a gasoline cut containing sulphur, preferably a gasoline cut from a catalytic cracking unit, with a boiling point range typically from about the boiling points of hydrocarbons containing 5 carbon atoms (C5) to about 220xc2x0 C. The end point of the gasoline cut depends on the refinery from which it originates and market constraints, but generally remains within the limits indicated above.
The process of the invention comprises separating the gasoline into two fractions: a light fraction (also termed the light fraction or light gasoline below) with an end point generally of about 160xc2x0 C. or less, preferably less than 140xc2x0 C. and more preferably less than 120xc2x0 C., a heavy fraction (also termed the heavy fraction or heavy gasoline below) constituted by the heavy fraction which is complementary to the light gasoline.
In general, the cut point is selected so as to maximise the olefin content in the light cut. This content can readily be determined, for example by means of a bromine number determination, which is generally available on-site.
Hydrodesulphurisation (also known as hydrotreatment) of the light gasoline is carried out over a nickel-based catalyst (described in a simultaneously filed patent application), and hydrodesulphurisation of the heavy fraction on a conventional hydrotreatment (hydrodesulphurisation) catalyst comprising a group VIII metal and a group VIB metal.
The desulphurised light and heavy cuts are then mixed. The effluent obtained can optionally be stripped to eliminate the H2S produced during hydrodesulphurisation.
It is also possible, and preferred particularly when the gasoline to be desulphurised contains polyolefins (dienes), to carry out selective hydrogenation of the gasoline before fractionation.